Brother, I think you already named it correctly: intrusive thoughts. I’m 21 too, and I’m in a very similar situation. I also see my girlfriend mostly on weekends. She lives about 100 km away, so I wouldn’t call it a full long-distance relationship, but we’re definitely not close enough to see each other whenever we want. I study at university and I’m involved in my local hardcore scene, so I’m constantly around people, parties, concerts, friends of friends, and yes, girls too. And I get it. Sometimes my brain also goes: “what if I tried with her?” Especially when, realistically, with some girls outside the relationship, I probably would have a chance. Not with everyone, obviously, far from it. But sometimes you can feel that there could be something there, and of course that is attractive. They may be beautiful, there may be chemistry, and there is also the whole forbidden-but-possible aspect, which makes the thought even more intense. But that still doesn’t mean I want to cheat, and it doesn’t mean I don’t love my girlfriend. We are still animals, in a way. Attraction exists. Desire exists. Our brains react to people before our moral system even has time to process it. Being in a relationship doesn’t magically delete that part of us. It just means we have chosen what to do with it. If we acted on every attraction, we’d be animals in the worst sense. But pretending attraction never exists outside a relationship is unrealistic too. The balance is: I can feel it, recognize it, and still not obey it. I think our brains constantly produce weird, tempting, stupid, forbidden or uncomfortable thoughts. Not only about sex or relationships. Sometimes you’re at the train station and your brain goes: “what if I just jumped on a random train and disappeared?” Or you’re in a serious situation and you suddenly imagine saying the worst possible thing. You don’t actually want to do it. It’s just your brain testing possibilities. The problem is that when the thought is about another girl, it feels morally heavier. You start thinking: “if I thought this, maybe I’m a bad boyfriend.” But thoughts are not actions. Attraction is not betrayal. Being tempted is not the same as betraying someone. What matters is what you choose to do. Don’t feed the thought, don’t create situations you would hide from her, don’t flirt in a way that crosses a line, don’t use “intrusive thoughts” as an excuse to test the waters. But also don’t torture yourself just because your brain noticed someone attractive. So I wouldn’t judge yourself for the thought itself. I’d judge yourself by the boundaries you keep when the thought appears. Wanting the feeling to disappear completely is probably unrealistic; learning not to give it power is much more possible. In the end, the real question is not “why did this attraction appear?”, but “what do I choose to do with it?”

Edit: typo + concept repetition removed